What is the working definition of "realms" when modern Tendai Buddhists talk about the mutual possession of realms (a la Śramaṇa Zhìyǐ's "yī niàn sān qiān / 一念三千")?

世尊在靈山會上拈華示眾眾皆默然唯迦葉破顏微笑世尊云The Lord dwelt at the Vulture Peak with the assembly and plucked a flower as a teaching. The myriad totality were silent, save for Kāśyapa, whose face cracked in a faint smile. The Lord spoke. 吾有正法眼藏涅槃妙心實相無相微妙法門不立文字教外別傳付囑摩訶迦葉。I have the treasure of the true dharma eye, I have nirvāṇa as wondrous citta, I know signless dharmatā, the subtle dharma-gate, which is not standing on written word, which is external to scriptures, which is a special dispensation, which is entrusted to Mahākāśyapa.नस्वातोनापिपरतोनद्वाभ्यांनाप्यहेतुतः
उत्पन्नाजातुविद्यन्तेभावाःक्वचनकेचन